NOT a single tonne of carbon was saved. In the short term, the planet will benefit not one jot. Some are calling it a betrayal of both science and the world's poor. Yet the climate conference in Durban, South Africa, which finished early on Sunday morning, did rewrite the rule book for fighting climate change in one way. It forced major developing nations like China, Brazil and South Africa to accept the principle of future binding targets on their greenhouse gas emissions for the first time.

A bleary-eyed Chris Huhne, the UK's climate change secretary, hailed the deal, which came after three overnight negotiating sessions and a 36-hour conference overrun. "This is the first time we have seen major economies commit to take action demanded by the science," he said.

The conference agreed that by 2015 governments would finalise a "protocol, legal instrument or an agreed outcome with legal force" that would impose targets on all major emitters, rich and poor. It will probably enter into force by 2020, when existing voluntary targets end, though that is not part of the official deal.

The bad news is that the deal is a post-dated cheque. It won't do anything to help the climate in the next decade - a decade that scientists say is critical to arresting global warming and turning the world's energy infrastructure towards low-carbon sources. Every year, countries spend about a trillion dollars on energy infrastructure, and right now coal is still the fuel of choice.

It is also far from clear what the promised binding targets will be - that question went undiscussed here. Most accept that the poorest nations will not face absolute cuts to their emissions. Instead, they will cut their "carbon intensity": the amount of CO2 emitted per unit of GDP, a reflection of how dirty their industries are.

But what of China? Along with fellow coal-burning superpower India, China held out longest against mentioning legal provisions in the Durban text. By 2020 it will be responsible for a quarter of the world's emissions and will probably have per-capita emissions as high as Europe.

With the legal targets not due to kick in till 2020 at the earliest, a key question is what will happen during the next decade. Voluntary action is now the only thing limiting emissions in most countries. And while the European Union agreed to accept a second phase of the Kyoto protocol that will limit its own emissions between now and 2020, few other industrialised nations will join in. Canada pulled out this week, joining other renegades: Russia, Japan and the US. After next year, the protocol will cover only around 15 per cent of global emissions.

That means voluntary targets will, in theory at least, have a big effect on global emissions in the coming years. In the last two rounds of annual climate talks, in Copenhagen, Denmark, and Cancún, Mexico, more than 80 countries - including major emitters not bound by the Kyoto protocol like Brazil, China, Indonesia and the US - pledged voluntary targets. Mostly, these pledges are not to cut emissions but carbon intensity.

In Durban, US chief negotiator Todd Stern said such promises were much more meaningful than the Kyoto protocol. But there are doubts. Independent modellers at Climate Analytics in Potsdam, Germany, say the pledges are wide open to governments cooking the books.

Their analysis of the loopholes (see "The problem with voluntary targets") suggests global CO2 emissions in 2020 could soar above the widely quoted 55 billion tonnes. That is far in excess of the 44 billion tonnes that the UN Environment Programme (UNEP) says is needed for a cost-effective route to staying below 2 °C of global warming.

Time is running out. However successful the deal felt early on Sunday, the brutal truth for climate negotiators is this: since 2007, when a "road map" to halt warming at 2 °C was agreed in Bali, Indonesia, they have spent four years on talks that have come to nothing. The plan for a deal to come into force when the Kyoto protocol expires in December 2012 sank without trace. The Durban agreement is essentially a pact to start again, with some added text about the legal nature of the future deal.

In the final hours, European negotiators in Durban tried to address the small matter of what happens in the next decade - the so-called "ambition gap". A working group made up of a small number of nations will now investigate ways to persuade countries to boost their voluntary pledges before 2020. It may also look for new ways of curbing emissions not currently covered by any targets, legal or otherwise - everything from international air travel and shipping to the soot from a billion African cooking stoves. If they can muster enough political will, all is not lost. There are ways to close the ambition gap (see chart), but nations must act now.

"I can't see anything in these negotiations that will prevent warming beyond 2 °C," said UNEP director Achim Steiner as he left Durban hours before the conference's conclusion. "To do that will require the world's carbon dioxide emissions to peak by 2020." Instead, we now have an agreement to agree on emissions cuts to begin in 2020, preceded by a voluntary period where nations do what they will. Whatever the diplomatic triumph, that is the bottom line for the planet.

The problem with voluntary targets

With meaningful UN deals proving so challenging, many argued in Durban that voluntary targets would be a smarter way to curb climate change.

But problems with plan B are becoming apparent. Chief among them is a lack of ground rules and the near-impossibility of policing whether governments are meeting their pledges, says Marion Vieweg of the climate modelling group Climate Analytics, in Potsdam, Germany.

The US, for instance, has promised to cut emissions by 17 per cent from 2005 levels without saying what their emissions were that year, says Vieweg. "They keep changing the data, especially for the contribution of forests and land-use changes."

China promised to cut its carbon intensity - emissions per unit of GDP - by 45 per cent by 2020, but hasn't said how much its economy could grow, something that will depend on imponderables of national accounting. Recent data suggest that by 2020 it could be emitting a billion tonnes more carbon dioxide than previously thought, says Vieweg.

Brazil promised to cut emissions by 36 per cent from business-as-usual growth. But earlier this year it sharply raised its business-as-usual emissions forecast, giving it an extra 18 per cent of emissions. The revision appears to be due to higher emissions from deforestation.

Even the European Union's promises are uncertain. Its east European members have unused emissions permits left over from the first period of the Kyoto protocol. If they are allowed to keep them for the next period, the EU could carry on increasing its emissions at the same rate it is now.

Another danger is that carbon offsets could be counted twice, with both rich and developing nations using the same projects to meet their pledges.

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